How to use initializeMiddleware of oas tools to add customized properties to the HTTP request?

Question

I am trying to replace the existing swagger tools in my project with oas-tools.

Source code shown as follows

const express = require('express');
const swaggerTools = require('swagger-tools');
const oasTools = require('oas-tools');
const { errorHandler } = require('errorHandlerPackage');

/**
 * takes paths to controllers folder and swagger route config file and returns swagger router
 * @method swaggerMiddleware
 * @param {config.swaggerDoc} - path to the swagger api spec json/yaml
 * @param {config.controllers} - path to the folder containing the controllers associated 
 *                                 to above swagger api spec json/yaml
 * @param {config.useOasTools} - bool, optional, a flag to determine if oas-tools is used or not
 * @returns swagger router associated to the paths in the swaggerDoc
 */
const swaggerRouterFactory = ({ controllers, swaggerDoc, useOasTools }) => {
    const router = express.Router({ mergeParams: true });
    const options = {
        controllers,
        useStubs: true
    };
    const _addCustomizedPropToRequest = () => {
        router.use((req, res, next) => {
            const { a, b } = req.headers;
            req.a = a;
            req.b = b;
            next();
        });
    }
    const _addSwaggerSettings = (middleware) => {
        router.use(middleware.swaggerMetadata());
        router.use(middleware.swaggerValidator());
        router.use(middleware.swaggerRouter(options));
        router.use(middleware.swaggerUi());
        router.use(errorHandler);
    }
    if (useOasTools) {
        oasTools.configure({
            controllers,
            strict: true,
        })
        oasTools.initializeMiddleware(swaggerDoc, router, (middleware) => {
            oasTools.initializeMiddleware(swaggerDoc, router, (middleware) => _addCustomizedPropToRequest());
            _addSwaggerSettings(middleware);
        });
        return router;
    }

    swaggerTools.initializeMiddleware(swaggerDoc, (middleware) => {
        swaggerTools.initializeMiddleware(swaggerDoc, (middleware) => _addCustomizedPropToRequest());
        _addSwaggerSettings(middleware);
    });
    return router;
};

module.exports = swaggerRouterFactory;

I can get the req.a and the req.b when I called my API through swagger-tools, but when I tried to call the API through oas-tools, I got undefined for the a and b. According to the oas-tools doc, to migrate from swagger-tools to oas-tools, I only need to pass the app as the second parameter to use the initializeMiddleware function. But this seems doesn't work for the _addCustomizedPropToRequest, the router.use(middleware.swaggerRouter(options)) works well.

Could anyone tell me what have I missed?